Telecommunication devices typically alert customers to an incoming call by providing ringing signals, dial tones and busy signals. A centrally-located alternating current ("AC") ringing signal generator provides the necessary power to drive a ringer or tone source to create the desired signals. Today, rotary generators, magnetic generators or electronic oscillators usually serve as ringing signal generators.
Older telephone ringers include a series-parallel winding having an armature that drives a bell clapper. The ringing signal generator drives the winding. The winding requires a certain minimum drive current to provide sufficient magnetomotive force to drive the bell clapper. Because the winding has a nonlinear response to drive current, the current waveform is asymmetrical and tends to be peaked in one polarity direction. Also, due to reactive impedances in the winding, the drive current tends to be out of phase with the drive voltage. Moreover, operation of the winding requires bidirectional power. More recent telephone ringers are completely electronic and, depending upon their design, may or may not require drive currents to operate.
Since today's wire-based telecommunications infrastructure can include both older and newer telephone ringers, a central ringing signal generator must be capable of providing power to a variety of different telephone ringers. Some telephone ringers present an active load; others are passive. Active loads may be purely inductive, capacitive or resistive or may be some combination thereof. Further, a well-designed ringing signal generator must be able to accommodate phase distortions while maintaining a uniform voltage waveform to the telephone ringers.
Today's infrastructure also employs direct current ("DC") biasing to detect if the handset of a particular telecommunication device has been removed from its cradle (i.e., off-hook). The telephone ringer is normally coupled to the ringing signal generator (providing power at, for instance, 48 volts); when the handset is removed from its cradle, the telephone ringer is shorted. The flow of DC current through the ringer therefore indicates that the telephone is off-hook.
A prevalent telephone ringer topology currently in use is generally referred to as a "ferro" topology. The ferro topology is characterized by a converter having a transformer with a gap. A switching circuit on a primary side of the transformer alternately transmits opposite phases of the input power to provide power to the ringer. A positive temperature coefficient ("PCT") resistor provides a current-limiting function to protect the ringer from any overcurrent resulting from the saturation of the transformer due to the DC current. A primary limitation of the ferro topology is that it does not allow the recycling of power from the output to the input power source, resulting in inefficiencies in the converter. Additionally, the ferro topology requires a transformer that is large and heavy.
Another telephone ringer topology is described in U.S. Pat. No. 4,866,587, which issued on Sep. 12, 1989 to Wadlington and is entitled "Electronic Ringing Signal Generator." Wadlington discloses a converter topology that allows the recycling of the output power back to the input source (i.e., a bidirectional power flow). The foregoing reference is herein incorporated by reference. The converter of Wadlington, however, allows the recycling of power only as to one phase, either positive or negative, but not as to both phases. Another limitation of the converter of Wadlington is that the recycling of the power back to the source is not accomplished independently. Therefore, additional control circuitry is necessary to perform this function.
The Unitrode Corporation of Merrimack, New Hampshire, provides a four quadrant flyback converter topology in its Unitrode UCC3750 Source Ringer controller. This topology allows the recycling of power in both positive or negative voltage modes and also in both positive and negative current modes (so-called "four quadrant" operation). A major limitation of Unitrode's topology is that the recycling power process is not truly continuous. Thus, when the Unitrode controller is configuring its switches after sampling power, distortions appear in the controller's output waveform, indicating discontinuities.
Accordingly, what is needed in the art is a converter topology that permits bidirectional four-quadrant power flow and overcomes other limitations of the prior art.